The reliability of modern electrical equipment is closely related to the efficacy of the soldering process used to connect the large number of electrical components making up the integrated circuit or electronic device. Wetting of the surfaces of the metal components to be joined is an essential prerequisite to the creation of a soldered joint. Sufficient wetting of the components to be joined can occur only if the metal oxides typically residing on the surface of the metals to be joined are removed. Fluxing agents are typically employed to improve the efficiency of the soldering operation.
Three major types of fluxing agents are typically used wherein the selection of the particular type depends upon the particular application. The major types are water soluble fluxing agents, natural rosins and activated rosins. Water soluble fluxes made from inorganic acids, organic acids, amine hydrohalides and the like, are reasonably effective in removing oxides from metal surfaces to be soldered but typically leave a residue on the metal surface which may corrode the parts following soldering and impact the reliability of the apparatus. Moreover, the cleaning and removal of water soluble fluxes typically creates waste water disposal concerns. While hydrocarbon and chloro-fluorocarbon solvents can be used to remove the residue, such solvents are not totally effective and their use adds complexity and cost to the process as well as posing a potential threat to the Earth's environment.
Natural rosins typically do not contribute to the corrosion of the soldered components but are somewhat weak in fluxing activity. Finally, activated rosins have a stability similar to natural rosins and do not corrode the fluxed metal surfaces at room temperature. However, fully activated fluxing agents often generate corrosive gases at soldering temperatures which can harm the substrate. Moreover, the residue of the activated fluxing agents can combine with moisture to produce corrosive acids which may shorten the life of the soldered joint. Soldering processes utilized in the production of microelectronic components such as printed circuit boards typically utilize non-activated rosin fluxes treated with activators such as acids, bases and salts which typically leave a residue requiring a post-cleaning step to remove such harmful residues.
A cover gas wave soldering system marketed by Seitz & Hohnerlein (SEHO), Kreuzwertheim, West Germany, employs a fluxing agent comprising formic acid in admixture with a nitrogen cover gas. Formic acid, in the presence of metals and metal oxides and at a temperature of about 150.degree. C., breaks down into carbon dioxide and hydrogen wherein the reducing power of the resultant hydrogen is added to the reducing power of the amino acid fluxing agent utilized.
A need exists for improved fluxing agents and processes for employing such fluxing agents in order to increase the reliability of the joining operation by promoting effective wetting of the solder to the workpiece prior to soldering and for reducing the incidence of post-soldering failure due to corrosion and fatigue of the workpieces. Moreover, elimination of the post-cleaning step for removal of residue from the soldered workpiece would greatly reduce processing costs and eliminate the use of potentially environmentally hazardous solvents such as chloro-fluorocarbons.